When I used to buy eggs I would frequently get fertile ones (in the regular box of eggs, not the ones marked "fertile") at various times of the year.
All egg houses also have massive hatching facilities...and contracts to deliver x number of eggs per day / week / whatever. And we all know that hens can be unpredictable with laying habits, even production breeds. So I have surmised that during the winter, when egglaying can decrease, the commercial egg producers walk over to the hatching sheds and fill out their contract orders wtih fertile eggs. Same during the summer when the heat causes decreases in production or in the spring when the hatching house is overloaded with fertile eggs.